Experimental evidence for factors maintaining plant species diversity in aNew England salt marsh

Understanding the processes that maintain species diversity patterns is vit al for conserving and managing communities. In this paper, we examine the e ffect of plant species interactions on the maintenance of plant species num ber across intertidal zones characterized by different soil conditions in a New England salt marsh community. In this system, plant species number is low in the high intertidal and lower middle intertidal, and high in the upp er middle intertidal. To understand the causes of this unimodal pattern of change in species diversity, we experimentally tested for the influence of competition, facilitation, and physical factors across this physical gradie nt. We established plots with and without plant neighbors, transplanted pla nt species into these plots, and measured mortality, leaf area, and flower number for a single growing season. We found that competition plays a more important role in the high intertidal habitat by decreasing leaf area and f lower production and causing 100% mortality of one of the species tested. I n contrast, physical factors are critical in the lower middle intertidal, c ausing 100% mortality of three of the four species tested, with or without neighbor interactions. In the upper middle intertidal, direct positive inte ractions were important to the higher species number. Three of the four spe cies had 100% mortality without neighbors but minimal mortality or leaf are a loss with neighbors. These positive effects were due to one particular fa cilitator species, Juncus gerardi L., which ameliorates the soil conditions that develop in its absence thus allowing other species to co-occur. Witho ut Juncus, our data predict that plant species number would not be as high in the upper middle intertidal. We show that the rise in species number in the upper middle intertidal is dependent on three co-occurring conditions: the absence of a competitive dominant, less harsh physical conditions than the lower middle intertidal, and the presence of a facilitator species.